The present invention relates to an exposure control system for use in photography, and more particularly, to a system which provides exposure information for a photography operation under ambient light and flash light.
When a flash light is used for taking a photograph, it is necessary in an ordinary prior art arrangement to adjust a diaphragm aperture of the camera lens by using the information of the distance to the object and the amount of light to be emitted from the flash light. Recently, there has been proposed a flash light that can be adjusted to change its light amount by the use of information of distance to the object and aperture value of the camera.
In either one of the above systems, it is so designed that when the photograph is taken with the flash light, the ambient light is not taken into consideration, on the assumption that the ambient light, when compared with flash light, has little effect on the exposure to the film surface.
However, this is not true in all the circumstances. Sometimes, the ambient light has an effect so strong that if it is disregarded, the film may be over exposed.
To solve the above problem, there have been proposed devices which, when a photograph is taken with flash light, take the light intensity of ambient light and the flash light into consideration. One such device is disclosed in U.S. Pat. Re. No. 28,025 to Tomoji MURATA et al. entitled "Automatic Control Device For Electronic Flash" and reissued May 28, 1974. According to this patent, ambient light intensity is previously measured. When the flash light is emitted, the reflected light from the object is incessantly measured and integrated. When the integrated level comes into a predetermined relation with the information of previously measured ambient light intensity, shutter speed, aperture value, and film sensitivity capable of providing a proper exposure on the film surface, the flash light is cut off. The device of MURATA et al., however, can not previously set or predict the flash light amount. The flash light amount can not be determined and known till the completion of the flashing operation.
Another device is disclosed in U.S. Pat. No. 3,836,920 to Takashi UCHIYAMA et al. entitled "Exposure Control System For Flash Photography" and issued Sept. 17, 1974. According to this patent, a ratio of contrast under the ambient light between the primary photographic object and secondary photographic object (background) is measured, before taking a photograph, and a ratio of distance to the object between the primary and secondary is set. When either one of shutter speed or flash light amount is set, the other will be automatically controlled in such a manner as to set the contrast ratio equal to 1. The diaphragm value is also controlled to expose the film with a proper light intensity. A similar device is disclosed in U.S. Pat. No. 4,188,104 to Takashi UCHIYAMA et al. entitled "Automatic Control Device For Electronic Flash" issued Feb. 12, 1980. In this patent, a procedure similar to that disclosed in U.S. Pat. No. 3,836,920 is carried out except for one difference. The difference is that the distance to the secondary object is presumed to be infinite. The above two patents measure the contrast between the primary and secondary objects, the ratio therebetween is designed to be always set to 1, though such a ratio "1" is not believed to be always good. Further, inputs and arithmetic procedures would be complex.
A further device is disclosed in U.S. Pat. No. 4,079,389 to Akihiko HASHIMOTO et al., entitled "Daytime Strobo Apparatus For Cameras With Electric Shutters" and issued Mar. 14, 1978. According to this patent, an integrator is employed which starts to integrate light intensity of the object by means of a TTL system when the shutter is opened. When the integrated value reaches the first predetermined value which is determined in accordance with a set lighting contrast, flash light is emitted. Thereafter, when the integrated value reaches the second predetermined value, the flash light is cut off and the shutter is closed. Since the light intensity is measured by means of the TTL system with the diaphragm being stopped down, the information of the diaphragm value is included in the information obtained by the light measuring operation. This device also fails to foresee information of the flash light amount prior to the taking of the photograph.
In case of the above mentioned prior art, all the devices do not process the logarithmically converted value of the signals, but process linear values of the signals. Therefore, a wide dynamic range cannot be possible unless there is an undesirable increase of the source voltage.